[LyX/master] Use macro LYX_USE_CXX11 in one more location

[Jean-Marc Lasgouttes]

commit c444fddc0b40160ba867130dd0280eac16399818
Author: Jean-Marc Lasgouttes <lasgout...@lyx.org>
Date:   Fri May 22 12:41:30 2015 +0200

    Use macro LYX_USE_CXX11 in one more location
    
    This is a follow-up to commit 329eae56.

diff --git a/src/tex2lyx/Parser.h b/src/tex2lyx/Parser.h
index 9778765..489b132 100644
--- a/src/tex2lyx/Parser.h
+++ b/src/tex2lyx/Parser.h
@@ -125,7 +125,7 @@ public:
 
        iparserdocstream(idocstream & is) : is_(is) {}
 
-#if (__cplusplus > 199711L)
+#ifdef LYX_USE_CXX11
        /// Like std::istream::operator bool()
        /// Do not convert is_ implicitly to bool, since that is forbidden in 
C++11.
        explicit operator bool() const { return s_.empty() ? !is_.fail() : 
true; }